A battery electric vehicle, or BEV, is a type of electric vehicle (EV) that uses chemical energy stored in rechargeable electric vehicle batteries (EVBs), also known as “battery packs” and “traction batteries.” BEVs use electric motors and motor controllers instead of (or in addition to) internal combustion engines for propulsion. A battery-only electric vehicle or all-electric vehicle derives all of its power from its battery packs while a pluggable hybrid electric vehicle derives part of its power from its battery packs and part of its power from an internal combustion engine. Examples of BEVs include automobiles, trucks, golf carts, trains and ships.
Traditional electric vehicle batteries differ from starting, lighting and ignition (SLI) batteries because they are designed to give power over sustained periods of time. Deep cycle batteries are used instead of SLI batteries for these applications. Traction batteries must be designed with a high ampere-hour capacity. Battery packs for BEVs are characterized by their relatively high power-to-weight ratio, energy-to-weight ratio and energy density.
Traction batteries come in several types including, by way of example, lead-acid, nickel metal hydride, and lithium ion traction batteries. Lead acid are generally the cheapest traction batteries available, but have a low energy density. Nickel metal hydride, while being less efficient in charging and discharging than lead acid, have a much greater energy density and can operate well over an extended lifetime (e.g. 10 years of service or 100,000 vehicle miles). Lithium ion batteries have a high energy density, good power density and high charge/discharge efficiency but suffer from short cycle lives and significant degradation with age. New types of lithium ion batteries have been developed that sacrifice energy and power density to provide greater fire resistance, environmental friendliness, rapid charging, and longer lifespans.
Battery pack designs for Electric Vehicles (EVs) are complex and vary widely by manufacturer and specific application. However, they all incorporate a combination of several mechanical and electrical component systems which perform the basic required functions of the pack. Moreover, battery packs incorporate many discrete cells connected in series and parallel to achieve the total voltage and current requirements of the pack. Battery packs can contain several hundred individual cells.
To assist in manufacturing and assembly, the large stack of cells is typically grouped into smaller stacks called modules. Several of these modules will be placed into a single pack. Within each module the cells are welded together to complete the electrical path for current flow. Modules can also incorporate cooling mechanisms, temperature monitors, and other devices. In most cases, modules also allow for monitoring the voltage produced by each battery cell in the stack by a battery management system, or “BMS.” The battery pack also contains a variety of other sensors, such as temperature and current sensors, which are monitored by the BMS. BMS can also be responsible for communications with the world outside the battery pack.
When used in battery electric vehicles, the operation of battery packs are currently designed to meet ISO 26262 Functional Safety standards, incorporated herein by reference. Functional Safety is the part of overall safety of a system or piece of equipment that depends upon the system or equipment operating correctly in response to inputs, including the safe management of likely operator errors, hardware failures and environmental changes. The ISO 26262 Functional Safety standard is entitled “Road vehicles—Functional safety”, and is an adaption of the Functional Safety standard ICE61508 for Automotive Electric/Electronic Systems, incorporated herein by reference.
The circuitry of certain BMS devices has been integrated into integrated circuits (ICs). To meet ISO26262 safety requirements related to automotive products and customer system reliability goals, it is necessary to diagnose failures within the IC and the application circuit that affect the safety goal of the application. These failures must be detected within the fault tolerant time of the voltage sources being monitored (various battery types, supercaps and fuel cells primarily).
Safety Integrity Level (SIL) is defined as a relative level of risk reduction provided by a safety function, or to specify a target level of risk reduction. IEC61508 provides for Safety Integrity Levels. ISO26262 provides a framework for Automotive Safety Integrity Levels (ASILs). ASIL ratings are A, B, C, or D, with D being the highest end of the A-D scale. Safety is very important in battery electric vehicle applications due to the chemicals, high voltages and high energy of battery packs and because of the potential for fire and/or explosion caused by the improper operation or failure of battery packs.
An important aspect of the ASIL standard is to monitor for failure both in the battery pack and in the devices monitoring the battery pack. For example, the cells are monitored for an over-voltage condition which could cause fires and/or the release of noxious fumes. However, if the monitoring device itself fails or malfunctions, dangerous conditions with respect to the battery pack can be missed.
One way of addressing the problem of the failure of a monitoring device is redundancy. For example, multiple battery pack monitoring devices can be used in parallel to monitor the health of a battery pack. The outputs of the multiple monitoring devices can be compared and if they are the same it can be assumed with some degree of certainty that the monitoring devices are working properly. However, if the outputs of the multiple monitoring devices are different, it can be assumed that one or more of the monitoring devices are not working properly and a “fault” condition can be initiated.
A problem with using multiple monitoring devices is cost and system complexity. Also, the nature of a fault condition is not known in that the system only knows that one or more of the redundant monitoring devices is not working properly. These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.